Semiconductor light emitting device

ABSTRACT

A semiconductor light emitting device includes a hetero-configuration having an active layer, a first clad layer, and a second clad layer, the active layer being interposed between the clad layers. The active layer emits light when charge carriers are injected. The first and second clad layers keep the injected charge carriers in the active layer. The hetero-configuration is interposed between a first and a second electrode. The semiconductor light emitting device further includes a dense defect-injected layer. This layer is provided between the first electrode and the hetero-configuration. The dense defect-injected layer is made of material more fragile than the hetero-configuration. The dense defect-injected layer prevents defects injected into the hetero-configuration.

This application is a Continuation application of Ser. No. 08/578,980filed Dec. 27, 1995, the entire contents of which are incorporatedherein by reference and claims the benefit of priority from the priorJapanese Application No. 6-325713 filed Dec. 27, 1994.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor light emitting device.Particularly, this invention relates to a semiconductor light emittingdevice with lose crystal defects and higher performance.

FIG. 1 shows a conventional semiconductor light emitting device at itscross section. This semiconductor light emitting device consists of: asemiconductor substrate 11 of n-type gallium arsenide (GaAs); atransparent buffer layer 12 of n-type GaAs; a reflective layer 13consisting of laminated two layers of indium aluminum phosphate(InAlP)/GaAs (InAlP on GaAs); a lower clad layer 14 of n-type InGaAlP;an active layer 15 of undoped InGaAlP; an upper clad layer 16 of p-typeInGaAlP; a transparent current diffusing layer 17 of p-type AlGaAs; acontact layer 18 of p-type GaAs; an upper electrode 19 and a lowerelectrode 20.

The bluffer layer 12 prevents faults from being produced due tocontamination of the surface of the semiconductor substrate 111 and alsoprevents the active layer 15 from being infected with the defects.

The reflective layer 13 reflects light emitted by the active layer 15 sothat the emitted light does not enter the buffer layer 12 and thesemiconductor substrate 11 made of light absorbent material. For thisreason, the reflective layer 13 consists of semiconductor layers ofInAlP and GaAs laminated with each other in a predetermined thickness.The layers of InAlP and GaAs have different refractive indices to theemitted light. The lower and upper clad layers 14 and 16 keep chargecarriers injected into the active layer 15 to achieve high luminousefficiency.

The active layer 15 consists of In_(1-y) (Ga_(1-x)Al_(x))P_(y). Thecomponents “a” and “y” and the layer construction determine energy gap.The active layer 15 emits light of wavelength corresponding to theenergy gap when the injected carriers recombine with each other.

The current diffusing layer 17 diffuses current thereacoss to take outthe emitted light through whole region of the layer 17 not only directlybelow the upper electrode 19.

The current diffusing layer 17 is made of transparent material (p-typeAlGaAs) that has a small absorbing coefficient to the emitted lightwavelength.

The contact layer 18 makes better ohmic contact between the currentdiffusing layer 17 and the upper electrode 19;

The upper electrode 19 is a p-type electrode of Au layer which containszinc. Thorough the upper electrode 19, a current is injected into a chipof the semiconductor light emitting device. The upper electrode 19spreads the current over entire region of the semiconductor chip.Further, the upper electrode 19 is formed so as not to scatter theemitted light. The upper electrode 19 also acts as a bonding pad.

The lower electrode 20 is an n-type electrode of Au formed as a layerwhich contains germanium. The lower electrode 20 drains the current.

Another conventional semiconductor light emitting device is disclosed byJapanese Patent Laid-Open NO: 4 (1992)-212479. The conventional deviceis a light emitting diode with double hetero-configuration. In thisdevice, an InGaAlP active layer is interposed between two clad layers.

Such a device with the InGaAlP active layer has required advancedepitaxy aiming at epitaxial growth with better crystallization, or fewercrystal defects. This epitaxial growth achieves higher devicereliability. Further, such a light emitting device is fabricated with amolding material of low resin stress. The low-resin stress materialreduces decrease in luminescence after the light emitting device isdriven.

However, it is very hard to keep crystal defects to a minimum in alllayers grown by epitaxy. Device selection for quality in accordance withthe number of crystal defects in all epitaxy-grown layers lowers deviceproduction yields. Further, low- and high-temperature degradation tests,after packaging the devices with molding resin, tend to produce muchdegradation in the resin packaged devices.

SUMMARY OF THE INVENTION

A purpose of the present invention is to provide a semiconductor lightemitting device with high reliability and production yields.

The present invention provides a semiconductor light emitting deviceincluding: a hetero-configuration having an active layer that emitslight when charge carriers are injected, a first clad layer, and asecond clad layer, the active layer being interposed between the cladlayers, the first and second clad layers keeping the injected chargecarriers in the active layer; a first and a second electrode, thehetero-configuration being inter-posed between the electrodes; and afirst dense, defect layer, provided between the first electrode and thehetero-configuration, the first dense defect layer being made ofmaterial more able to absorb crystal defects and prevent defectextension and migration than the hetero-configuration, the first densedefect layer preventing defects from extending or migrating into thehetero-configuration.

The device may further include a second dense defect layer, providedbetween the second electrode and the hetero-configuration. The seconddense defect layer is made of material more able to absorb crystaldefects and prevent defect extension and migration than thehetero-configuration. The second dense defect layer prevents defectsfrom extending or migrating into the hetero-configuration.

The hetero-configuration may be a double hetero-configuration in whichthe active layer is undoped, and the first and second clad layers aredoped for a specific conductivity type.

The device may further include a current diffusion layer, providedbetween the first electrode and the first dense defect layer. Thecurrent diffusion layer diffuses current applied through the firstelectrode.

The device may further include a semiconductor substrate providedbetween the second electrode and the hetero-configuration and a bufferlayer provided on the semiconductor substrate. The buffer layer preventsdefects from being generated in the semiconductor substrate and theexpansion of the defects into the active layer.

The present invention further provides a semiconductor light emittingdevice including: a hetero-configuration having an active layer thatemits light when charge carriers are injected, a first-clad layer, and asecond clad layer, the active layer being interposed between the cladlayers, the first and second clad layers keeping the injected chargecarriers in the active layer; a first and a second electrode, thehetero-configuration being interposed between the electrodes; and adense defect layer, provided between the first electrode and thehetero-configuration, the dense defect layer being made of material moreable to absorb crystal defects and prevent defect extension andmigration than the hetero-configuration, the dense defect layerpreventing defects from extending or migrating into thehetero-configuration; a current diffusion layer, provided between thefirst electrode and the dense defect layer, the current diffusion layerdiffusing current applied through the first electrode; a contact layer,provided between the first electrode and the current diffusion layer,the contact layer making ohmic contact between the first electrode andthe current diffusion layer; a semiconductor substrate, provided betweenthe second electrode and the hetero-configuration; a buffer layer,provided on the semiconductor substrate, the buffer layer preventingdefects from being generated in the semiconductor substrate and theexpansion of defects into the active layer; and a reflective layer,provided on the buffer layer, the reflective layer reflecting lightemitted by the active, layer so that the emitted light does not enterthe buffer layer and semiconductor substrate.

The present invention further provides a semiconductor light emittingdevice including: a hetero-configuration having an active layer thatemits light when charge carriers are injected, a first clad layer and asecond clad layer, the active layer being interposed between the cladlayers, the first and second clad layers keeping the injected chargecarriers in the active layer; a first and a second electrode, the heteroconfiguration being interposed between the electrodes; a first densedefect layer, provided between the first electrode and thehetero-configuration, the first dense defect layer being made ofmaterial more able to absorb crystal defects and prevent defectextension and migration than the hetero-configuration, the first densedefect layer preventing defects from extending or migrating into thehetero-configuration; a current diffusion layer, provided between thefirst electrode and the first dense defect layer, the current diffusionlayer diffusing current applied through the first electrode; a contactlayer, provided between the first electrode and the current diffusionlayer, the contact layer making ohmic contact between the firstelectrode and the current diffusion layer; a second dense defect layer,provided between the second electrode and the hetero-configuration, thesecond dense defect layer made of material being more able to absorbcrystal defects and prevent defect extension and migration than thehetero-configuration, the second dense defect layer preventing defectsfrom extending or migrating into the hetero-configuration; and a bufferlayer, provided on the second electrode, the buffer layer preventingdefects from being generated in the semiconductor substrate and theexpansion of defects into the active layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional schematic illustration of a conventionalsemiconductor light emitting device;

FIG. 2 is a cross sectional schematic illustration of a preferredembodiment semiconductor light emitting device according to the presentinvention;

FIG. 3 is a graphical representation of variation of luminanceefficiency;

FIGS. 4A and 4B show fragmentary sectional views of the conventionalsample chip and that of the present invention;

FIG. 5 is a graphical representation of comparison to devicecharacteristics of the conventional sample chip and that of the presentinvention; and

FIG. 6 is a cross sectional schematic illustration of another preferredembodiment of a semiconductor light emitting device according to thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments will be described with reference to the attacheddrawings.

FIG. 2 shows a cross sectional schematic illustration of an embodimentof the semiconductor light emitting device according to the presentinvention. The layers of the same reference numerals as the layers shownin FIG. 1 function the same as those conventional device layers.

This semiconductor light emitting device includes a buffer layer 12 ofn-type GaAs; a reflective layer 13 consisting of laminated two layers ofindium aluminum phosphate (InAlP)/GaAs (InAlP on GaAs); a lower cladlayer 14 of n-type InGaAlP; an active layer, 15 of undoped InGaAlP; anupper clad layer 16 of p-type InGaAlP; a dense defect layer 30; acurrent diffusing layer 17 of p-type AlGaAs; and a contact layer 18 ofp-type GaAs. These layers are formed in order on a semiconductorsubstrate 11 of n-type gallium arsenide (GaAs). A doublehetero-configuration consists of the lower clad layer 14, the activelayer 15 and the upper clad layer 16. An upper electrode 19 is formedthe contact layer 18. A lower electrode 20 is formed beneath thesubstrate 11.

Either of the buffer layer 12 and the current diffusing layer 17 may beomitted. The reflective layer 13 and the contact layer 18 may also beomitted.

Compared to the conventional device of FIG. 1, the added feature of theembodiment of FIG. 2 is the dense defect layer 30 interposed between thep-type InGaAlP upper clad layer 16 and the p-type AlGaAs currentdiffusing-layer 17. The dense defect-injected layer 30 is made of 50nm-thick InP mixed crystal that is more able to absorb crystal defectsand prevent defect extension and migration than the p-type InGaAlP upperclad layer 16.

The semiconductor light emitting device is fabricated as follows:

The layers described above are grown epitaxially one by one on then-type GaAs substrate 11 formed on a semiconductor wafer as describebelow. These epitaxial growths are performed in a chemical vapordeposition (CVD) reaction chamber. A carrier gas (hydrogen) flows intothe CVD reaction chamber at a gas flow rate of 101/min. Thesemiconductor substrate 11 is annealed at a temperature in the range of720° to 870° C.

(1) Trimethylgullium (TMG) and arsenic hydride (AsH₃) flow into thechamber at a gas flow rate in the range of 20 to 400 ccm and 500 to 800ccm, respectively. Further, silicon hydride (SiH₄) flows at a gasvelocity in the range of 10 to 15 ccm for doping to form the n-type GaAsbuffer layer 12 on the substrate 11.

(2) Trimethylgullium and AsH₃ flow again at the same gas flow rates instep (1) to form a GaAs layer on the buffer layer 12. Further,trimethylindium (TMI), trimethylaluminum (TMA) and phosphorus hydride(PH₃) flow at a gas velocity in the range of 0.5 to 0.8 ccm, 10 to 300ccm and 250 to 400 ccm, respectively, to form an InAlP layer on the GaAslayer to form the InAlP/GaAs reflective layer 13.

(3) Trimethylindium, TMG, TMA and PH₃ flow at the same gas flow rates inthe above steps. Further, SiH₄ flows at the same gas flow rate in step(1) to form the n-type InGaAlP lower clad layer 14 on the reflectivelayer 13.

(4) Trimethylgullium, TMG, TAM and PH₃ flow at the same gas flow ratesin the above steps to form the undoped InGaAlP active layer 15 on thelower clad layer 14.

(5) Trimethylgullium, TMG, TAM and PH₃ flow at the same gas flow ratesin the above steps. Further, dimethlylzinc (DMZ) flows at a gas velocityin the range of 0.3 to 0.5 ccm for doping to form the p-type InGaAlPupper clad layer 16 on the active layer 15.

(6) The susceptor temperature is decreased by 100° C. Trimethylaluminumand PH₃ flow at the same gas flow rates in the above steps to form thedense defect layer 30 of 50 nm-thick InP mixed crystal on the upper cladlayer 16.

(7) The chamber temperature decreased by 100° C. in step (6) isincreased to the original temperature at which the process is executedin steps (1) to (5). At this temperature, TMA, TMG and AsH₃ flow at thesame gas flow rates in the above steps. Further, DMZ flows at the samegas flow rate in step (5) for doping to form the p-type AlGaAs currentdiffusing layer 17 on the dense defect layer 30.

(8) Trimethylgullium and AsH₃ flow at the same gas flow rates in theabove steps. Further, DMZ flows at the same gas flow rate in step (5)for doping to form the p type GaAs contact layer 18 on the currentdiffusing layer 17. And,

(9) A reverse-sided lapping operation thins the substrate 11. The upperand lower electrodes 19 and 20 are deposited on the contact layer 18 andthe thinned substrate 11, respectively. The semiconductor wafer on whichthe above multiple layers were laminated was diced and molded to obtainmany chips of semiconductor light emitting devices (FIG. 2). Each chipwas of 400×400 μm² in area and 200 μm in height. Also produced were thechips of the conventional semiconductor light emitting devices (FIG. 1)of the same size as the present invention.

These semiconductor light emitting devices were tested for luminanceefficiency. A forward current of 20 mA of 5 volts was supplied to eachdevice to find out initial luminance efficiency and luminance efficiencyafter 500 hours have elapsed. These tests were conducted for determiningthe degradation rate of the semiconductor light emitting devices of thepresent invention and the conventional devices.

Fifty sample chips were selected per sample lot from the semiconductorlight emitting devices of the present invention and also from theconventional devices to determine the initial luminance efficiency andluminance efficiency after 500-hour elapsing. The dense defect layer 30of 50 nm-thick InP mixed crystal was grown for the devices of thepresent invention.

FIG. 3 is a graphical representation of variation of the luminanceefficiency after 500 hours have elapsed indicated by the relativeefficiency ratio (ratio of initial luminance efficiency/luminanceefficiency after 500 have elapsed). Each dot depicts an average survivalrate for 50 samples per lot (A, B, C, D, E, and F). The upper and lowerends of each bar depict the maximum and minimum survival rates,respectively. FIG. 3 teaches that the sample chips of the presentinvention (II) have a higher survival rate than the conventional samplechips (I). Further, FIG. 3 teaches that the sample chips of the presentinvention have nearly the survival rate for the lots D, E, and F.

The conventional sample chips (lot B) that had the worst survival rateswere analyzed by cathode luminescence technique.

This technique revealed an un-luminous crystallization fault 40 called adark line as shown in FIG. 4A. FIG. 4A shows a fragmentary crosssectional view of the conventional sample device chip of FIG. 1. Thedark line crossed the current diffusing layer 17 from the devicesurface. Further, the dark line penetrated into the upper clad layer 16,active layer 15, and lower upper clad layer 14.

The destruction of the active (light, emitting) layer 15 by theun-luminous crystallization fault 40 was deemed to cause the lowsurvival rates, and corresponding high levels of degradation. The darkline (fault 40) extended towards the active layer 15 from directly belowa bonding wire (not shown) fixed on the upper electrode 19 of FIG. 1. Itis believed that: wire bonding caused damage to the device surface; thedamage expanded due to heat and resin stress; and the expanded damagepenetrated into the device as the un-luminous crystallization fault 40that damaged the active layer 15.

The sample device chips of the present invention were also analyzed bythe cathode luminescence technique. This technique revealed anun-luminous crystallization fault 40 a as shown in FIG. 4B. FIG. 4Bshows a fragmentary cross sectional view of the sample device chip ofthe present invention of FIG. 2. The un-luminous crystallization fault40 a produced due to wire bonding crossed the current diffusing layer17.

However, contrary to the conventional sample device chip of FIG. 4A, theun-luminous crystallization fault 40 a stopped in the 50 nm-thick densedefect InP layer 30 that is the feature of the present invention. Theun-luminous crystallization fault 40 a did not reach the active layer 15and upper clad layer 16. The faults 40 a are absorbed or impeded in thedense defect layer 30 is believed to be the reason for the higherluminance efficiency of the device chips of the present invention. Moreprecisely, the dense defect layer 30 was deemed to prevent theun-luminous crystallization fault 40 a from or migrating into thecladding layer 14 or active layer 15 due to heat and resin stress. Thedevice chips of the present invention were thus protected from defectmigration or extension from external areas into the clad and activelayers of the device.

FIG. 4B, the fragmentary cross sectional view of FIG. 2, furtherschematically depicts prevention of secondary generated defects fromextending or migrating into the active layer 15 and upper clad layer 16by the dense defect-injected layer 30.

This advantage was provided by the use of the Inp mixed crystal layerfor the dense defect layer 30. Besides Inp mixed crystal, use of GaP,InGaP, IAlP, AlP, and AlAs mixed crystals as the dense defect layer 30is also contemplated.

However, InGaAs mixed crystal did not work well for the dense defectlayer 30. This was noted by observing the boundary of the InGaAs densedefect layer and InGaAlP layers as the active and clad layers with crosssection Transmission Electron Microscopy (TEM).

The observation revealed that: enough defects were not provided in theInGaAs layer acting as the dense defect layer 30; the InGaAs layer couldnot sufficiently disperse the secondary defects traveling or migratinginto this layer and due to bonding damage; and a part of the secondarydefects were injected into the InGaAlP upper clad layer 16.

The observation further revealed that reduces the effects of aun-luminous crystallization fault 40 a can be achieved by providingdefects in the dense defect-injected material and not in the InGaAlPlayer.

Moreover, the observation revealed as shown in FIG. 5 that: the desiredreduction in defect effects can be achieved when the defect density (thenumber of defects) of the dense defect injected layer 30 is 10⁴/cm² ormore; the difference in lattice constant is 10⁻² or more between thedense defect layer 30 and InGaAlP upper clad layer 16; and the densedefect layer 30 is preferably 10 nm or more in thickness.

FIG. 6 shows a cross sectional schematic illustration of anotherembodiment of the semiconductor light emitting device according to thepresent invention. The layers of the same reference numerals as thelayers shown in FIG. 2 function the same as those conventional devicelayers. And hence explanation of those are omitted here.

This embodiment does not require the semiconductor substrate 11 of FIG.2. An upper dense defect layer 30 a is formed between the transparentcurrent diffusion layer 17 and the upper clad layer 16. Further, a lowerdense defect layer 30 b is formed between the transparent buffer layer12 and the lower clad layer 16.

These upper and lower layers 30 a and 30 b restrict crystal defectdamage to the active region of the double hetero-configuration thatconsists of the n-InGaAlP lower clad layer 14, InGaAlP active layer 15and p-InGaAlP upper clad layer 16. Further, the layers 30 a and 30 brestrict crystallization faults-being passed into the current diffusionlayer 17 and buffer layer 12, respectively. The crystallization faultsare generated mostly due to internal stress caused by thermal expansionand shrink-age when the devices are molded. The lower dense defect layer30 b can restrict generation of crystallization faults.

The semiconductor devices of the two embodiments include the doublehetero-configuration. This configuration consists of the n-type InGaAlPlower clad layer 14, p-type InCaAlP upper clad layer 16, and undopedInGaAlP active layer 15 interposed between the two clad layers.

According to the preferred embodiments of the invention, semiconductordevices, particularly light emitting devices, can be obtained with highreliability, long lifetime, high yield rates, and of reasonable price.The light emitting device includes a double or singlehetero-configuration that consists of a pair of clad layers and anInGaAlp active layer interposed between the clad layers. Duringepitaxial growth of this device, a dense defect layer is formed on orbeneath the hetero-configuration. Or; two dense defect-injected layersare formed on and beneath the hetero-configuration. The dense defectlayer is made of material of two or three mixed crystals. The mixedcrystals are a combination of elements selected from the groupconsisting of In, Ga, Al, P, and As. The elements for the combinationare different in lattice constant of 10⁻² or more. Further, the densedefect layer includes defects of 10⁴/cm² or more. Such a dense defectlayer prevents secondarily generated defects from migrating or extendingas un-luminance crystallization faults into the important InGaAlP active(light emitting) layer.

As described above, the present invention provides a semiconductordevice configuration including at least a first layer with a firstfunction, a second layer with a second function, and a third layerinterposed between the first and second layers. The third layer is adense defect-injected layer made of material that is more able to absorbcrystal defects and prevent defect migration and extension than thesecond layer. The third layer disperses or absorbs a dark line (theun-luminous crystallization fault in the embodiments) that wouldotherwise cross the first layer and reach the second layer. The thirdlayer thus restricts the extension or migration of crystallizationfaults. The present invention is therefore useful for any semiconductordevices with a layer of specific function that should be protected fromcrystallization faults.

1. A semiconductor light emitting device, comprising: a first clad layerof a first conductive type; an active layer formed on the first cladlayer; a second clad layer of a second conductive type formed on theactive layer; a defect layer formed on the second clad layer; a firstelectrode electrically connected to the first clad layer; and a secondelectrode electrically connected to the second clad layer.
 2. Asemiconductor light emitting device of claim 1, wherein the defect layerhas a higher defect density than the second clad layer.
 3. Asemiconductor light emitting device of claim 1, wherein the defect layeris 10⁻² or more different in lattice constant from the second cladlayer.
 4. A semiconductor light emitting device of claim 2, wherein thedefect layer is 10−2 or more different in lattice constant from thesecond clad layer.
 5. A semiconductor light emitting device of claim 1,wherein the defect layer is 10 nm or more in thickness.
 6. Asemiconductor light emitting device of claim 2, wherein the defect layeris 10 nm or more in thickness.
 7. A semiconductor light emitting deviceof claim 1, wherein the defect layer is 10 nm or more in thickness.
 8. Asemiconductor light emitting device of claim 2, wherein the defect layeris 10 nm or more in thickness.
 9. A semiconductor light emitting device,comprising: a first clad layer of a first conductive type; an activelayer formed on the first clad layer; a second clad layer of a secondconductive type formed on the active layer; first means provided on thesecond clad layer protecting the second clad layer from a newly injecteddefect; a first electrode electrically connected to the first cladlayer; and a second electrode electrically connected to the second cladlayer.
 10. A semiconductor light emitting device of claim 9, furthercomprising second means provided between the first clad layer and theactive layer for protecting the first clad layer from a newly injecteddefect.
 11. A semiconductor light emitting device of claim 9, furthercomprising a current diffusion layer provided between the first meansand the second electrode.
 12. A semiconductor light emitting device ofclaim 10, further comprising a current diffusion layer provided betweenthe first means and the second electrode.
 13. A semiconductor lightemitting device of claim 9, wherein the newly injected defect isun-luminous crystallization fault.
 14. A semiconductor light emittingdevice of claim 10, wherein the newly injected defect is un-luminouscrystallization fault.
 15. A semiconductor light emitting device ofclaim 9, wherein the un-luminous crystallization faults is extended fromupper side of the first means.
 16. A semiconductor light emitting deviceof claim 10, wherein the un-luminous crystallization faults is extendedfrom bottom side of the second means.